In rotating anode tube X-ray sources electrons are accelerated between a cathode and the rotating anode across a kilo-Volt potential difference so that they impact onto the focal track of the rotating anode. In the process, X-rays are generated, which are said to emanate from a “focal spot” at the point of electron beam impact on the rotating anode. Due to the energy dissipated in the impact, the rotating anode's surface may be modified in the process, and small pits and cracks may begin to appear.
As a consequence, the X-ray yield and spectrum of the tube may change over the course of its lifetime, which may have an impact on the application in which the tube is used. The trend in modern X-ray tube applications is for the instantaneous power applied to the rotating anode to increase, commensurately with the increase of the gantry speed (in a CT system).
Furthermore, the shrinking of the detector cells and the focal spot (to allow better spatial resolution), and the widening of the anode angle (for widened detector coverage) lead to an increased power density at the focal spot of rotating anode X-ray tube. The rate of alteration of the rotating anode's surface caused by the electron beam is only likely to increase in the future.
In US 2009/0067578 A1, a rotating anode is described with a structure comprising slits, which can be detected if a structure placed on the anode passes the focal spot. Thus, properties of the focal spot can be determined from changes of the detected signal during operation of the X-ray tube.